in ocaml Use options to re-implement `assoc` with a more truthful type:
let rec assoc : (key: 'k) (l : ('k * 'v) list) : 'v =
        begin match l with
        | [] -> failwith "Not_found"
        | (k2, v) :: tl -> if key = k2 then v else assoc key tl
        end